JP2643445B2 - High frequency heating equipment - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a high-frequency heating device such as a microwave oven, and more specifically, to a so-called magnetron inverter power supply using a semiconductor switch element in a power supply circuit of the magnetron. It relates to improvement of the device.

2. Description of the Related Art Conventionally, a high-frequency heating device using a power supply circuit of this kind
Although various types have been proposed, a typical one is a voltage resonance type inverter circuit having a parallel resonance circuit as shown in FIG.

This circuit rectifies by a bridge diode 2, an inductor 3, and a capacitor 4 of a commercial power supply 1 to form a pulsating DC voltage source (power supply circuit), and includes a resonance capacitor 5, and a step-up transformer 6 also serving as a resonance inductor. A voltage resonance type inverter circuit is constituted by the parallel resonance circuit, the transistor 7 and the diode 8, and the high voltage output of the step-up transformer 6 is rectified by the capacitor 9 and the diodes 10 and 11 and supplied to the magnetron 12. 13,14,15
Are the primary, secondary and heater windings of the step-up transformer 6, respectively, and 16 is a control circuit for driving the transistor 7.

In this power supply circuit, the transistor 7 is connected to the control circuit 16
The conduction time Ton by the so-called pulse width control
(Ton 'is controlled substantially by the control of Ton'). That is, the switching operation by the transistor 7 and the diode 8 is performed by the gate signal VGE as shown in FIG. 6 (a) and the voltage waveform VCE as shown in FIG. 6 (C). TOFF is determined by the circuit constant of the parallel resonance circuit, while the amount of power converted by the inverter circuit is determined by TON. Therefore, the amount of power supplied to the magnetron 12 is determined by TON, and The power supplied to the magnetron 12 increases as the operating frequency decreases. The control circuit 16 controls the TON to adjust the magnitude of the output of the magnetron 12 and to stabilize the output against fluctuations in the power supply voltage.

Problems to be Solved by the Invention The switching state of the transistor 7 ideally has a waveform as shown in FIG. 6B, but actually has a switching waveform as shown in FIG. That is, as shown in FIG. (A), at time t-t _1, the gate voltage VGE becomes to 0V, and the collector current of the transistor 7
IC decreases rapidly. Then, the voltage VCE between the collector and the emitter rises as shown in FIG. However, in a switching circuit that handles a large amount of power (for example, about 1 kW or more), it is impossible for a semiconductor switching element such as a transistor to realize an ideal switching operation. = become the time of t _3. In particular, when a switching element having a relatively large tail current (Ita) such as an insulated gate bipolar transistor is used as a semiconductor switching element, the switching loss at the time of turn-off due to the tail current becomes particularly noticeable.

In particular, in the case where the power supply device is further downsized by increasing the switching frequency, the loss of the transistor 7 becomes a problem due to the switching loss, and the practical maximum switching frequency cannot be limited. According to the experiments by the inventors, the conversion power of the inverter circuit was set to 1 KW, and under sufficient heat radiation conditions (conditions for mounting an insulated gate bipolar transistor on a radiator that can be regarded as an infinite heatsink and forcibly cooling) As shown in FIG. 8, when the operating frequency f0 of the inverter was increased, it was found that thermal runaway occurred at about 70 kHz to 80 kHz, leading to destruction. This is because the time during which the so-called tail current Ita flows (Δt = t ₃ −t ₂ ) is 1 to ₂ μs even at room temperature, and since this time Δt has a positive temperature coefficient, excessive loss occurs. If it does, the resulting temperature rise will lead to runaway. Therefore,
Although this operating frequency limit varies depending on the size of the package of the transistor 7 and the size of the chip, if economical design conditions are emphasized, it is considered that the operating frequency limit exists at about 70 kHz to 80 kHz. It has been difficult to realize a working inverter power supply.

For this reason, an inverter circuit that operates at a high frequency of 100 kHz or higher cannot be realized, and the power supply cannot be made smaller, lighter, and lower in cost. Was difficult.

The present invention solves such a conventional problem,
An object of the present invention is to provide a high-frequency heating device having a low-cost power supply device.

Means for Solving the Problems In order to solve the problems of the related art, the present invention has the following configuration.

That is, a power supply unit obtained from a commercial power supply or a battery, a semiconductor switch element having a self-commutation function, a series resonance circuit including a resonance capacitor and a resonance inductor, and boosting the resonance voltage of the series resonance circuit to supply the boosted voltage to the magnetron A step-up transformer, and a control unit that controls the semiconductor switch. The oscillator that oscillates at a frequency lower than the resonance frequency of the series resonance circuit, and a conduction time of the semiconductor switch that receives a signal from the oscillator , And an output circuit that drives the semiconductor switch element with a signal from the timer circuit, and the conduction time is longer than a half cycle of the resonance current of the series resonance circuit, and one cycle. It is configured to be shorter.

Further, a reverse bias detection circuit for detecting a reverse bias state of the semiconductor switch element is provided, and an output pulse of the output circuit is cut off by an output of the reverse bias detection circuit.

Further, a diode for rectifying the output of the step-up transformer is provided, and the rectified output of the diode is supplied to the magnetron. A diode current detecting means for detecting a current flowing through the diode, and an output of the diode current detecting means is provided. Is compared with a set value, and an error amplification control circuit that controls the oscillator frequency of the oscillator based on the error signal is provided, and the oscillator is controlled so that the diode current becomes a predetermined value. .

Operation With the above configuration, the present invention has the operation described below.

That is, a power conversion circuit is configured by the series resonance circuit and the semiconductor switch element, and the timer circuit provided in the control unit has a self-commutation function in which a drive pulse having a time width between a half cycle and a single resonance cycle is provided. With the configuration for supplying to the semiconductor switching element, it is possible to realize a power conversion circuit capable of turning off the semiconductor switching element in a state where the current flowing through the semiconductor switching element is substantially zero. It is possible to operate at a frequency one digit higher than that of the technology and to control ON / OFF of the semiconductor switching element having the self-commutation function without fail.

Also, by detecting the reverse bias state of the semiconductor switch element and cutting off the output pulse of the output circuit, even if the constant of the series resonance circuit greatly changes due to the temperature characteristics of the magnetron, capacitor, inductor, etc. In addition, a drive pulse having a time width between a half cycle and one cycle of the resonance cycle can be supplied to the semiconductor switching element having a self-commutation function, and in any case, the current flowing through the semiconductor switching element is substantially zero. A power conversion circuit that can be turned off in a state can be realized, and the above-described operation can be realized.

Furthermore, by detecting the current flowing through the output rectifier diode of the step-up transformer and performing feedback control on the oscillation frequency of the oscillator based on the error signal between the detection signal and the set value, the output of the power conversion circuit is stabilized. The high frequency output of the magnetron can be stabilized against power supply voltage fluctuations and the like.

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a circuit diagram of a high-frequency heating device showing one embodiment of the invention, and the components having the same reference numerals as those in FIG. 5 are the corresponding components.

The output of the commercial power supply 1 is rectified by the diode bridge 2 to be a full-wave rectification-like voltage waveform output. This output is a series resonance circuit consisting of an inductor 17 acting as a constant current power supply, a capacitor 18, a step-up transformer 19, a switching circuit consisting of an insulated gate bipolar transistor (IGBT) 7 and a diode 8, a resistor 20 and a capacitor. The power is supplied to a power conversion circuit (inverter) composed of a snubber circuit composed of 21 and is converted into high-frequency power of 100 to 300 kHz. Since this high-frequency power is generated as a high-frequency voltage in the primary winding 22 of the step-up transformer 19, high-frequency high-voltage and high-frequency low-voltage power are induced in the secondary winding 23 and the heater winding 24, respectively. The DC voltage power is supplied to the magnetron 12 after being rectified by the diodes 25 and 26 and the capacitors 27 and 28, while the high-frequency low-voltage power is directly supplied to the cathode of the magnetron 12 to heat the cathode. Current waveforms and voltage waveforms flowing through the IGBT 7 and the diode 8 are as shown in FIGS. 2 (a) and 2 (b), respectively. By the control unit 29 described later
The IGBT 7 is driven by the gate voltage waveform VGE shown in FIG.

When the switch 30 such as a relay is closed, the control unit 29 outputs a resistor 31, a diode 32, a capacitor 33, and a zener diode.
Operation is started by the formation of a control power supply consisting of 34. The voltage controlled oscillator 35 is activated, and its output activates the timer circuit 36. The timer circuit 36 can be easily constituted by, for example, a monostable multivibrator having a reset terminal, and the voltage controlled oscillator 35 can be easily realized by an astable multivibrator having a voltage control terminal. The output of the timer circuit 36 is sent to the output circuit 37 and subjected to impedance conversion, and then supplied to the IGBT 7 via the resistor 38. As is apparent from FIG. 2, when the gate pulse VGE is supplied to the IGBT 7, the collector current Ic thereof starts to flow in a resonant manner as shown in FIG. t ₁ hour has passed Ic becomes zero, the next diode current Id flows, the Id also becomes zero to ₂ hours after t. IGBT
The gate pulse voltage VGE for driving the 7 longer than the t _1, and that a short pulse width ton than t _2, IG
It is extremely important for the safe operation of BT7. If shorter than ton is t _1, will be off the IGBT7 the period when Ic is flowing, occurs a large turn-off loss as in the conventional art. Conversely, if it is longer than t ₂ , a so-called commutation failure phenomenon is caused, resulting in a short-circuit current flowing through the IGBT 7 and destroying it. Specifically, when using a switching element having a self-commutation feature, such as IGBT, shorter than the resonant period t ₂ of the conduction time ton resonant circuit, and a long as conditions than the resonance half period t ₁ is safe It is extremely important and indispensable to ensure reliable operation and operate at high efficiency and high frequency.

The reverse bias detection circuit 3 in FIG. 1 detects the point in time at which Id starts to flow (after elapse of time t ₁ ) in FIG.
The timer circuit 36 is reset so as to cut off the output pulse VGE after a delay of Δt. Due to the reverse bias detection circuit 39, even if the characteristics of the magnetron 12, the capacitor 18, the transformer 19, etc. are greatly changed and the circuit constant of the series resonance circuit is changed, the pulse width (that is, conduction time) ton of the gate pulse VGE is Always longer than t ₁ and
The condition of being shorter than t ₂ can be satisfied, and a stable and low-loss switching operation can be realized. Of course, the reverse bias detection circuit 39 is not always necessary. For example, when the variation range of the circuit constant of the series resonance circuit due to the change in the characteristics of the magnetron 12, the capacitor 18, the transformer 19, etc. is relatively small, the count time of the timer circuit 36 is previously set to the conduction time t in FIG.
If it is set to be on, even if there is some variation in the circuit constant of the series resonance circuit, during the period when Id is flowing (that is, the period when Ic is zero and VCE is zero) The gate pulse VGE can be turned off. Therefore, in this case, only the timer circuit 36 such as a simple monostable multivibrator is required, and a simple and inexpensive configuration can be achieved.

Reference numeral 40 denotes a current transformer for detecting a current flowing through the diode 26. Since this diode current is about half of the anode current (proportional to the radio wave output) flowing through the magnetron 12, a signal corresponding to the anode current can be obtained with a small amount of current detection. Therefore, a small current transformer can be used. The output of the current transformer 40 is sent to the error amplifier 41, which controls the oscillation frequency of the voltage controlled oscillator 35 so that the output of the current transformer 40 becomes a set value. That is, the current transformer
When the output of 40 is smaller than the set value, error amplifier 41 controls so as to increase the oscillation frequency of voltage-controlled oscillator 35, and conversely, when the output is larger than the set value, performs so-called negative feedback control so as to decrease the oscillation frequency. The error amplifier 41 can be realized very easily by using a well-known operational amplifier. By detecting the diode current and performing negative feedback control in this way, it is possible to easily realize stabilization of the radio wave output with respect to the fluctuation of the voltage of the commercial power supply.

With the above configuration, it is possible to realize an inverter circuit that can operate at a frequency that is about one digit higher, which has been difficult in the past. For example, a conventional inverter circuit (FIG. 5)
In the case of converting 1KW power using the IGBT7, even when using the IGBT7 that generates a tail current of 1 to 2 μs, the configuration of the present invention realizes a high-frequency operation of about 100 kHz to 300 kHz and significantly reduces the size of the power supply device. Thus, the weight can be reduced, and as a result, a significant cost reduction can be achieved.

FIG. 3 is a more detailed embodiment of the reverse bias detection circuit 39 in FIG.

FIG. 3 is a more detailed embodiment of the reverse bias detection circuit 39 in FIG.

In FIG. 3, the same reference numerals as those in FIG. 1 denote corresponding components, and the voltage between both terminals of the collector terminal 43 and the emitter terminal 44 of the IGBT 7 (that is, the voltage between both terminals of the diode 8) is equal to that of the resistor 44. Is supplied to the comparator 45 via the.

The output of the comparator 45 is a resistor 46 and a capacitor 47
Is connected to the reset terminal R of the timer circuit 36 via the delay circuit constituted by the delay circuit, the timer circuit 36 is reset with a delay of a predetermined delay time Δt when the IGBT 7 is reverse biased. Therefore, as shown in FIG. 2, the gate pulse VGE ends during the period when Id is flowing, that is, during the period when the IGBT 7 is reverse-biased, and the turn-off loss of the IGBT 7 can be made extremely small. . Reference numeral 48 denotes a protective zener diode for limiting the input voltage to the comparator 45.

FIG. 4 is a more detailed embodiment of the control circuit 29,
Those having the same reference numerals as those in FIG. 1 are the corresponding components.

In the figure, the output of the current transformer 40 is a diode
49, Capacitor 50, Resistor 51 Converts to voltage, Operational Amplifier 52, Reference Voltage 53,5
4. The error is amplified by an error amplifier composed of resistors 57 to 58 and sent to an oscillation frequency control terminal 6 of an oscillator 59 (NE555 timer IC). 60 and 61 are resistors, and 62 is a capacitor. As is well known, the oscillation frequency is determined by these constants and the input voltage to the terminal 6. The output of the oscillator 59 is a monostable multivibrator that generates a pulse for a time determined by the time constant of the resistor 63 and the capacitor 64.
65 (NE555 timer IC). Therefore, an output pulse having a frequency corresponding to the oscillation frequency of the oscillator 59 and having a pulse width determined by the monostable multivibrator 65 is output from the monostable multivibrator 65 to the CMOS buffer 66. Since the output of the CMOS buffer 66 is configured to be supplied to the gate of the IGBT 7, the switching of the IGBT 7 is performed so that the current flowing through the diode 26 detected by the current transformer 40 becomes the value set by the reference voltage 53. The frequency is controlled. 67
Is a capacitor.

As described above, the control circuit with a very simple configuration
The current flowing through the diode 26 can be kept constant, and therefore, the current flowing through the magnetron 12 is also controlled to be constant, so that its radio wave output can be controlled to be substantially constant.

Effects of the Invention As described above, according to the present invention, a power supply unit obtained from a commercial power supply or a battery, a semiconductor switch element having a self-commutation function, a series resonance circuit including a resonance capacitor and a resonance inductor, An oscillator for boosting a resonance voltage of a resonance circuit and supplying the boosted transformer to a magnetron, and a control unit for controlling the semiconductor switch, wherein the control unit oscillates at a frequency lower than a resonance frequency of the series resonance circuit; A timer circuit that receives a signal from an oscillator and counts a conduction time of the semiconductor switch; and an output circuit that drives the semiconductor switch element with a signal from the timer circuit, wherein the conduction time is equal to a resonance of the series resonance circuit. The current flowing through the semiconductor switch element is longer than half the current and shorter than one cycle. Can realize a current conversion circuit that can be turned off in a state of substantially zero, significantly reducing the switching loss of the device, operating the semiconductor switch at an order of magnitude higher frequency than the conventional technology, and The semiconductor switching element having the self-commutation function can be reliably turned on and off. Therefore,
Realizing an inverter circuit that operates at a high frequency of 100 kHz or higher while meeting economical design conditions, making it possible to reduce the size, weight, and cost of the power supply unit. Can be provided.

Also, by detecting the reverse bias state of the semiconductor switch element and cutting off the output pulse of the output circuit, even if the constant of the series resonance circuit greatly changes due to the temperature characteristics of the magnetron, capacitor, inductor, etc. A drive pulse having a time width between a half cycle of the resonance cycle and one cycle can be supplied to the semiconductor switching element having a self-commutation function, and in any case, the current flowing through the semiconductor switching element is substantially zero. It is possible to realize a power conversion circuit capable of turning off the power supply circuit, to ensure sufficiently high performance stability against fluctuations in environmental conditions and manufacturing conditions, and to reduce the size and size of the power supply device described above. Light weight, low cost, and compactness can be realized.

Furthermore, by detecting the current flowing through the output rectifier diode of the step-up transformer and performing feedback control on the oscillation frequency of the oscillator based on the error signal between the detection signal and the set value, the output of the power conversion circuit is stabilized. It is possible to provide a power supply device that can stabilize the high-frequency output of the magnetron against fluctuations in power supply voltage and the like, and that has greatly promoted the above-described small size, light weight, low cost, and compactness.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a high-frequency heating device showing one embodiment of the present invention, and FIGS. 2 (a), 2 (b) and 2 (c) show the collector current Ic of an insulated gate bipolar transistor of the device, respectively. FIG. 3 is a waveform diagram, a waveform diagram of the collector voltage VCE, and a waveform diagram of the gate voltage VGE, FIG. 3 is a circuit diagram of a more detailed embodiment of the reverse bias detection circuit 39 of the device, and FIG. FIG. 5 is a circuit diagram of a conventional high-frequency heating device, and FIGS. 6 (a), (b) and (c) are insulated gate type devices of the same device, respectively. FIGS. 7 (a) and 7 (b) show a gate voltage waveform diagram, a collector current waveform diagram, and a collector voltage waveform diagram of a bipolar transistor, respectively, showing a detailed synthesis of the collector current and the diode current of the insulated gate bipolar transistor of the same device. Waveform diagram, FIG. 8 is a characteristic diagram showing a relationship between a rise value of a collector temperature of a transistor and an operating frequency in the same device. 1,2 ... power supply section (1 ... commercial power supply, 2 ... diode bridge), 7 ... semiconductor switch element, 18, 22 ... series resonance circuit (18 ... resonance capacitor, 22 ... step-up transformer), 22 …… Step-up transformer, 25,26 …… Diode, 29
…… Control unit, 35 …… Oscillator, 36 …… Timer circuit, 37…
... output circuit, 39 ... reverse bias detection circuit, 40 ... diode current detection means, 41 ... error amplification control circuit.

Continuation of the front page (72) Inventor: Tsuyoshi Kusunoki 1006 Kazuma Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP-A-62-86692 (JP, A)

Claims (3)

(57) [Claims] 1. A power supply section obtained from a commercial power supply or a battery, a semiconductor switch element having a self-commutation function, a series resonance circuit including a resonance capacitor and a resonance inductor, and boosting a resonance voltage of the series resonance circuit. An oscillator for oscillating at a frequency lower than a resonance frequency of the series resonance circuit; and an oscillator for oscillating the control unit at a frequency lower than a resonance frequency of the series resonance circuit. A timer circuit that counts the conduction time of the semiconductor circuit, and an output circuit that drives the semiconductor switch element with a signal of the timer circuit, and the conduction time is longer than a half cycle of the resonance current of the series resonance circuit, and , A high-frequency heating device configured to be shorter than one cycle. 2. The high-frequency heating device according to claim 1, further comprising a reverse bias detection circuit for detecting a reverse bias state of the semiconductor switch element, wherein an output pulse of the output circuit is cut off by an output of the reverse bias detection circuit. apparatus. 3. A diode for rectifying the output of a step-up transformer, a rectified output of the diode is supplied to a magnetron, a diode current detecting means for detecting a current flowing through the diode, and a diode current detecting means. An error amplification control circuit that compares an output with a set value and controls an oscillation frequency of the oscillator based on the error signal, and controls the oscillator so that the diode current has a predetermined value. The high-frequency heating device according to (1).